Method and device for spectroscopy of the optical emission of a liquid excited by a laser

ABSTRACT

A method for spectroscopy of the optical emission of a liquid excited by a pulsed laser focused on the surface thereof is such that the area of analysis is scanned by a laminar discharge of gas whose velocity and section are such that it is possible to remove the residues of the plasma suspended in the gas, resulting from a first laser pulse, before the subsequent laser pulse occurs.

The present invention relates to a process and to a apparatus forspectroscopy of the optical emission of a liquid excited by a laser.

It is known that the analysis of a compound by optical emissionspectroscopy can use an energy input applied to the compound analyzed bymeans of a laser beam producing radiations proper to various componentsof the compound, thus making it possible to identify the latter andtheir respective concentrations.

More precisely, this energy input produces a plasma made up of thechemical elements present in the analyzed compound, the luminousradiation of which is made up of rays of frequencies inherent in thenature of the components, whereas their intensity is determined by theconcentration of the latter.

Such a process makes it possible to perform quick analyses since thedetermination of the concentration of several elements in the liquidbeing performed simultaneously.

Moreover, it requires minimum preparation of the compound and it permitsobtaining an analysis resolution for detecting the components atconcentrations amounting to as little as one particle per million (ppm).

Furthermore, this process permits generating a minimum of effluents orwastes, since only the small amount of compound being analyzed beforeneeds to be considered regarding its recycling or its elimination.

However, such analysis is complex when the compound analyzed isphysically modified by the impact of the laser beam. This is so when thecompound is a liquid and the laser beam is applied to its surface, asdescribed in the document [1] entitled, “panoramic laser-inducedbreakdown spectrometry of water” by Messrs. Charfi and Harith, publishedin Spectrochimica Acta Part B: Atomic Spectroscopy, Jul. 31, 2002, vol.57, No. 7, pp. 1141 to 1153.

In fact, as indicated in this document, the impact of one pulse of laserbeam on the surface of a liquid produces, on the one hand, splasheswhich attenuate by their opacity the following pulses of the light beamand which are pollutants to the optical system measuring the radiationresulting from the interaction and, on the other hand, wavelets andshock waves at the surface of the liquid being analyzed, which defocusthe beam.

Now, some of such splashes arrest the essential of the light energybefore it interacts with the jet of liquid being analyzed, which causesmodifications of the optical emissions measured, although thecomposition of the liquid analyzed is constant.

For their part, the shock wavelets and waves at the surface of theliquid being analyzed defocus the laser beam, which also modifies theradiation emitted by the surface impacted by the light beam on thisliquid.

In other words, the splashes outside of the jet and the disturbances atthe surface increase the separation measured between successiveanalyses, consequently reducing the accuracy of the analysis.

To limit these inhomogeneities it is possible, as described in thedocument previously cited, to optimize the conditions of manipulation bytilting the laser beam with respect to the surface of the liquid so asto limit the impact of the beam on the surface.

Furthermore, a low frequency of repetition or recurrence, of the orderof 0.2 Hz, is utilized to limit again the wavelets at the surface of theliquid analyzed, whereas the analyses on solids, which do not have theseproblems, are performed at recurrence frequencies of 10 to 20 Hz.

Such surface problems can interfere with analyses such as the one citedby Nai-Ho Cheung and Edward S. Yeung in the document [2] entitled“Distribution of sodium and potassium within individual humanerythrocytes by pulsed laser vaporization in a sheath flow,” publishedin Analytical Chemistry 1994, 66, pp. 929 to 936.

In this document there is proposed an apparatus 100 (FIG. 1) permittingthe analysis, by laser excitation or ablation coupled with opticalemission spectroscopy, of liquid 110 issued from a cell, this liquidpresenting the difficulty of being available in too small a quantity topermit correct focusing of the laser beam on its surface.

To permit the analysis of this liquid 110, it is transmitted bycapillarity in a duct 112 coming in contact with the walls of a duct oflarger size 114, this second duct 114 transmits a liquid 116 whichtransports the liquid 110 that issued from the cell.

The duct 112 brings the liquid 110 being analyzed against the wall ofthe duct 114 such that, by capillarity, the liquid 110 being analyzed isat the surface of the liquid 116 that issued from the duct 114.

Thus, an analysis of the compounds situated at the surface of the liquid118, liquid mixture 110 and 116, leads to an analysis of the liquid 110.

To perform an analysis of a liquid without having to deal with theproblems mentioned above, it is known to generate the plasma radiatinginto the interior of the liquid being analyzed, as described by David A.Cremers, Leon 3. Radziemski, Thomas R. Loree, in the document [3]entitled, “Apparatus and method for spectrochemical analysis of liquidsusing the laser spark,” in U.S. Pat. No. 4,925,307 published 15 May1990.

In this document its authors mention the problems mentioned above inconnection with the analysis of a liquid, inherent in the focusing ofthe laser beam on the surface of the liquid and proposing the analysisof this liquid by generating, with the aid of a first laser, a plasma inthe interior of the liquid being treated, that is, by focusing the laserbeam inside of the liquid.

In a second step a second laser focused into the plasma produces theemission of light, the analysis of which constitutes the measurement.

Such a procedure involves the problem of requiring a precise positioningof the optics and a complex synchronization of the laser beams.

In other words, this process requires perfect mechanical stability ofthe system and presents great complexity and cost.

Furthermore, it is known that optical analysis by spectroscopicbeginning with laser excitation can be improved when the productanalyzed, i.e., receiving the laser beam, is in a specific gaseousenvironment.

For example, in document [4] entitled, “Determination of colloidal ironin water by laser-induced breakdown spectroscopy” presented by YoshiroIto, Osamu Ueki, Susumu Nakamura in Analytica Chimica Acta 199 (1995),pp. 401-405, the authors compare the properties of a helium, air orargon environment of a liquid to improve the spectroscopy of theemissions produced by the laser beam excitation of the latter.

In this document the authors cited describe the use of an apparatus 200(FIG. 2) comprising a duct 202 carrying a liquid 206 which is latersurrounded by a gas 208 whose effect on the emission of the rays fromthe plasma generated by laser in the liquid 206 is studied. The laserbeam is orthogonal to the surface of this liquid.

By making the liquid being analyzed to flow in jet form, this apparatusenables the laser beam to strike a different portion of this liquid oneach laser pulse, which limits the effect of the wavelets and shockwaves.

Furthermore, the analyses are made without problem of contamination ofwalls of the container of the cells, since the liquid is analyzed onlyat the outlet of the duct.

According to this document it was found that an argon (Ar) or helium(He) environment had different effects on the intensity of the signals;the argon increasing their intensity whereas helium reduced it.

This document also discloses that the temperature of the plasma,important to the quality and the accuracy of the signal analyzed couldbe kept high in spite of any low thermal conductivity of the gassurrounding it.

The invention results from the finding that in all the former techniquesdescribed above, the plasma resulting from the interaction of a laserbeam pulse with the jet of liquid under analysis violently hurlsmicrodroplets at very high velocity in all directions, which greatlyinterfered with the measurements and harms the accuracy of the analysis.

In fact, since microdroplets of liquid are in suspension around theplasma formed by a first laser beam, they disturb any new use of thelaser beam on the surface.

Since then, the various pulses of the laser beam applied to the surfaceof the liquid are attenuated erratically by the cloud of micro-dropletsprojected by a preceding pulse, and consecutive analyses of the liquidshow differences caused by these microdroplets. In other words, therepeatability and accuracy of the analyses are limited by thesemicrodroplets.

The present invention aims to remedy this difficulty.

More precisely, the invention consists of a process for the opticalemission spectroscopy of a liquid excited by a pulsed laser focused onits surface, characterized in that the area of analysis is swept by alaminar flow of gas having sufficient speed and cross section toeliminate the residues of the plasma suspended in the gas resulting froma first laser pulse before the following laser pulse takes place.

The gas thus a function of sweeping away the residues of a precedingplasma, and also of contention on the liquid whose surface isstabilized, which contributes also to the repeatability of the analyses.

Otherwise, if the gaseous environment is determined, and the focusing ofthe light rays is accomplished, no modification of the lens or of thearrangement of the installation is any longer necessary, even when theliquid being analyzed is changed, except, if such be the case, if one ofthese liquids has a viscosity very different from the others. Thisabsence of adjustments during analyses increases the repeatability andthe repeatability of the analyses.

Furthermore, it is preferable for the gas for eliminating residues to bechosen from among gases which improve the radiation generated when theanalysis is performed.

The invention thus concerns a method for optical emission spectroscopyof a liquid excited by a pulsed laser focused on its surface, a gasbeing arranged in the vicinity of the zone of analysis including thissurface, characterized in that the gas is given a laminar sweepingmovement parallel to the surface being analyzed, this movement havingsufficient velocity to eliminate the residues in this gas of the plasmaproduced by a preceding laser pulse and having a sufficient crosssection to eliminate the residues of the plasma that are suspended inthe gas.

In one example of embodiment, the gas disposed near to the liquid of thezone of analysis produces a contention effect on the free surface ofthis liquid.

The velocity of the gas is determined according to at least one of thefollowing characteristics of the liquid analyzes: its temperature, itsviscosity, its rate of flow, the turbulent or laminar nature of itsflow.

The section swept by the laminar flow of the gas is determined accordingto at least one of the following characteristics: speed of expansion ofthe plasma, rate of recurrence of the laser pulses, and accuracy of themeasurement.

Preferably the liquid is flowing in the zone of analysis.

In one embodiment the gas is carried into the zone of analysis by a ductsurrounding the duct carrying the liquid under analysis.

The gas used is, for example, argon or helium.

The radiation issued by the plasma of interaction between the liquidbeing analyzed and the laser beam is, in a preferred embodiment,collected colinearly with the laser beam.

The zone of analysis and the means for generating a jet of the liquid tobe analyzed and a jet of gas surrounding it can be removed into anair-tight chamber able to contain dangerous products or a hostileenvironment and confine them. In this case the colinearity of theradiation emitted by the plasma of interaction between the liquid beinganalyzed and the laser beam is particularly advantageous, for it permitsusing only one porthole for the enclosure.

Preferably, the laser beam is inclined with respect to the plane formedby the surface of the analyzed fluid at an angle other than 90 degrees.

It is preferable that, when the liquid is flowing, the point of impactof the laser beam on the jet is close to the outlet of the liquid from aduct. For example, this distance is between 5 and 15 mm for water. Infact, after a certain distance depending on the velocity of flow of theliquid, the jet becomes unstable and then diverges.

The invention also concerns an optical emission spectroscopy apparatusfor a liquid excited by a pulsed laser focused on the surface of thisliquid, characterized in that it comprises:

a laser able to generate pulses of coherent light with a power densityof at least 1 Gw/cm²,

means for generating a laminar jet of the liquid being examined, of alength of at least one centimeter,

means for generating a laminar jet of gas parallel to the surface of theliquid being analyzed, and in contact therewith,

means for focusing the laser beam into the zone of analysis, on thesurface of the jet of liquid being analyzed,

a means for collecting the light resulting from the interaction of thelight pulses of the laser with the jet of liquid being analyzed,

a spectroscope able to operate in the range of frequencies at which therays of emission of the liquid being analyzed are found, and arranged soas to receive the light of interaction collected by the bundle ofoptical fibers,

means for circulating the examined liquid in jet form, and

means for circulating the gas in jet form that must flow tangentially tothe analyzed liquid.

According to an embodiment, the means for gathering the light emitted bythe liquid being examined is such that this light is gathered colinearlywith the excitation laser beam,

and the apparatus has an air-tight enclosure in which the liquid to beanalyzed and the means for generating the laminar gas, jet,

the colinearity of the excitation laser beam and of the direction of thecollected light permit the use of a single porthole of the enclosure forthe laser beam and the collected light.

Other characteristics and advantages of the invention will appear withthe detailed description of an embodiment made hereinbelow as anon-restricting example, by referring to the annexed drawings in which:

FIGS. 1 and 2 already described represent known apparatus forspectroscopic analysis of optical emission using an excitation laser.

FIG. 3 represents in detail an ablation cell, i.e., an apparatus forspectroscopic analysis of optical emission using an excitation laser,according to the invention, and

FIG. 4 represents an embodiment of the apparatus shown in FIG. 3.

The apparatus 300 described below with the aid of FIG. 3 permits theperformance of a process, pursuant to the invention, of the analysis ofa liquid 301 by optical emission spectroscopy generated by means of alaser beam 303 focused on the surface of the liquid 301.

For this purpose, properties of speed and rate of flow are conferredupon a gas 309 in the analysis zone 304 comprising this liquid surface,such that the disturbances generated by the impact of the laser beam onthe surface of the liquid are minimized. The velocity and rate of flowof the gas 309 must be of values sufficiently high to eliminate themicrodroplets. However, this velocity and this rate of flow must notexceed a certain limit so as not to disturb the flow of the liquid jet.The pressure of the gas feeding the apparatus is controlled in order toadjust the speed and rate of flow.

The ablation cell 300, which can be discharged into an enclosure able tocontain and confine a hostile environment, includes a dud 302 directingthe liquid 301 to be analyzed to an analysis zone 304, that is, a zoneincluding the surface of the liquid 301 onto which a laser beam to bedescribed later on is focused.

This liquid duct 302 crosses a support 306 for connection to a dud 310bringing in gas 309, such as nitrogen or argon. This support 306 enablesthe gas 309 to be distributed all around the liquid dud 301 and to bringit out through a duct 313 of the same axis as dud 301 and surroundingit. This dud 313 exits through an orifice 313 ₁ with diameter D₁, andthe liquid duct 302 discharges through an orifice 302 ₁ of diameter D₂.Inside of the support 306 sealing means 312 are placed between thelatter and the liquid dud 302, at a level situated between the end ofthis dud 301 in the support and the inlet dud 310 in order to force thegas to escape toward the analysis zone 304.

Dud 302 is a Pasteur pipette having an outlet orifice 302 ₁ of adiameter D₂ of 0.1 mm, and the dud 313 is a tube of inside diameter D₁of 10 mm.

The velocity and rate of flow (the pressure in the example) of the gas309 must not exceed the limit beyond which this gas might deflect thejet of fluid 301 to be analyzed in zone 304, or make it fluctuate, whichwould defocus the laser beam and would make the analysis lose itsprecision. It is easy to determine this threshold experimentally foreach pair of liquid to be analyzed and gas. For example, when the liquidto be analyzed and the gas is air or nitrogen, the limit is 1 bar.

When the liquid is water and the gas is air or nitrogen, this effect isobtained by feeding the inlet duct 310 with gas under a pressure P_(gas)greater than the ambient pressure P_(ambient) of 0.15 at 1 bar, andpreferably 0.2 bar.

This effect of scanning must be adapted to the physical characteristicsof the solution and of the gas, and especially its viscosity. Forexample, if the liquid to be analyzed is an oil having a cinematicviscosity at 100 EF of 67.6 cst and the gas is air or nitrogen, thepressure required in the gas inlet duct 310 must reach 0.4 bar above theambient.

When the gas pressure satisfies these conditions, this gas stabilizesthe jet by a contention effect, and drives away from the analysis zone304 the microdroplets formed around the plasma, such that the latter donot disturb the action of the laser on the liquid when another lightpulse arrives, which is significant since these pulses are at least onesecond apart.

Such a reduction of the disturbances has made it possible to improve thesignal-to-noise ratio by about a factor of 100 with respect to the useof the gas at ambient pressure. This improvement is perceived, dependingon the modalities selected by the operator, both in a great improvementof the accuracy and repeatability of the measurements for the same rateof repetition of the laser pulses, and in a great improvement of therate of repetition of the laser pulses, to the degree that the laserpermits, and also in a simultaneous, but less important, improvement ofeach of these parameters.

In the analysis zone 304, the gas 309 in laminar flow eliminates themicrodroplets in suspension after the impact of a pulse of the laserbeam and stabilizes the surface of the liquid 301 under analysis.

Lastly, the apparatus described in this example presents arrangementsknown in closely related contexts. In particular, it is known that, fora liquid at rest, the laser beam can be tilted in relation to thesurface of this liquid at an angle other than 90°, so as to limit thedisturbances engendered by the laser beam. In the example, this angle isgreater than 60° and less than 90°.

Furthermore, since the liquid analyzed is flowing in the analysis zone,the bubbles formed in the liquid by the laser beam are removed from thiszone by the flow.

In this embodiment the liquid 301 is collected in a vessel 314 providedwith a duct 316 one end of which is introduced into the liquid 301 atthe start of operation.

The duct 316 is connected to a pump 418 (FIG. 4) such that it ispossible to recycle the fluid 301 so as to use but a limited amount ofliquid for performing the analyses.

Furthermore, the focus of the laser and of the optical system forrecording the emissions can be fixed throughout all of the analyses.Then the analysis apparatus is particularly stable, again improving therepeatability of the analyses.

As indicated before, the gas 309 permits obtaining a very repeatable andstable plasma. It is then possible to use repeat or recurrentfrequencies for the laser from 10 to 20 Hz, or more, for a period ofseveral minutes. This permits obtaining high spectroscopic accumulationperiods, and hence improving the signal-to-noise ratio.

The apparatus represented in FIG. 4 comprises a laser 402 emitting beamsat a fundamental wavelength of 1064 nm, to which is added a frequencydoubler bring this wavelength to 532 nm. It also comprises a glassdichroic mirror 404, a quartz dichroic mirror 406 and a convergent lens408 to aim and focus the laser beam on the surface of the liquid to beanalyzed in the analysis zone 304.

In this example, the laser 402 is an Nd-YAG laser of wavelength 1064 nm,reduced to 532 nm by a frequency doubler, and emitting pulses with aduration of seven nanoseconds. Any pulse rate of the order of 2 to 30 nsis also appropriate as long as the specific power delivered to the jetfor analysis is at least 1 Gw/cm².

Given the elimination from the gas of the residues of the precedingplasma and the stabilization of the surface of the liquid beinganalyzed, the laser can operate at a recurrent frequency of ten oftwenty Herz so as to perform a large number of analyses for a givenperiod, thus improving the repeatability of this analysis.

Furthermore, it should be pointed out that the quartz dichroic mirror406 permits transmitting analytical rays in the ultraviolet range.

The radiation emitted by the plasma in this analysis zone is guided upto an optical fiber bundle 420, which can be reduced to a single opticalfiber, by the convergent lens 408 formed of a single lens, the mirror406, then a convergent optic 410 thus formed of a single lens. Bundlesof fibers called split section/bundle transformers, which permitgathering an approximately circular light spot, and apply it almostwithout loss to the input slot of a spectrometer. The material of thesefibers should permit the transmission of all the rays emitted by theliquid being analyzed.

The bundle of optical fibers 420 is here reduced to a single fiber, ofsilica, of one millimeter diameter and about ten meters of length.

Unlike the apparatus of the prior art (FIG. 2), this fiber collects thespectrum of interaction radiated by the plasma along the same axis asthe laser beam incident on the zone 304, which contributes toestablishing the signal. In fact, this colinearity permits maintainingthe existence of the signal if the position of the point of impact ofthe laser beam varies under the effect of the plasma.

This characteristic maximizes the light gathered when the laser beam isnot perpendicular to the surface of the liquid. Moreover, it favors theuse of the apparatus in hostile environments, such as a vacuum or anuclear environment, by reason of the possibility of passing theexcitation laser beam and the light spectrum gathered through a one andonly port 407 of the protective and confining enclosure 409 representedby broken lines.

Application to radioactive solutions used in the nuclear power industryconstitutes a privileged application. In this case this enclosure 409represents the walls of a “hot cell” of the nuclear industry, and theport 407 is preferentially made of quartz.

The gathering of light by means of a fiber optic permits workingremotely and saves the user of the apparatus from having to be close tothe zone where the radioactive (or toxic, or of difficult access)solutions are handled. Thus, in the case where the analysis concernsdangerous products or has to be performed in a hostile environment, itis possible to situate the ablation cell 430, defined by the line 409,in the hazardous environment, while keeping the rest of the apparatus inan environment safe for the operators.

The possibility of performing analyses of different solutions withoutmaking any adjustment among these solutions is then particularlyadvantageous. Direct focusing on the spectrometer would be possible, butmore complicated to adjust.

This radiation is then analyzed by a spectrometer 422, such as a CzernyTurner spectrometer or a so-called scale spectrometer connected to acomputer 424 recording the emission spectra, so as to process thesedata.

The Czerny Turner geometry spectrometer makes it possible, with optimumadjustment, to scan a spectral range from 250 nm to 650 nm with aspectrum window for simultaneous access of 4 nm.

The scale spectrometer has the same resolution as the Czerny Turnergeometry spectrometer, but its spectrum window, with the adjustmentselected, covers a wavelength range of 200 to 850 nm.

Such a scale spectrometer, equipped with a CCD camera, preceded by alight intensifier, can be calibrated upon starting, since no moving partis present in the detector.

In this case, a pulse generator permits starting a window of time formeasuring the radiation recorded by the camera, with a delay selected inrelation to the laser pulse.

The spectrometer 422 is controlled with the aid of the computer 424which is equipped with a data acquisition and processing software.

The analysis of various solutions is of great importance in very manyindustrial domains, such as the pharmaceutical, electronic, and energyindustries and hostile environments. One of the privileged applicationsis the analysis of radioactive solutions in the nuclear energyprocesses.

1. Method for the optical emission spectroscopy of a liquid (301)excited by a pulsed laser (402) focused on its surface, characterized inthat the analysis zone (402) is scanned by a laminar flow of gas (309)having sufficient velocity and cross section to eliminate the residuesof the plasma in suspension in the gas and resulting from a first laserimpulse before the following laser impulse supervenes.
 2. Methodaccording to claim 1, characterized in that the laminer flow of gasachieves a contention effect on the free surface of the liquid. 3.Method according to claim 1 or 2, characterized in that the velocity ofthe gas is determined as a function of at least one of the followingcharacteristics of the liquid analyzed: its temperature, its viscosity,its rate of flow, the turbulent or laminar nature of its flow.
 4. Methodaccording to any one of claims 1 to 3, characterized in that the crosssection swept by the laminar flow of the gas is determined according toat least one of the following characteristics: speed of expansion of theplasma, rate of recurrence of the laser pulses, accuracy of themeasurement.
 5. Method according to any one of the foregoing claims,characterized in that the liquid is flowing in the analysis zone. 6.Method according to any one of the foregoing claims, characterized inthat the gas is carried into the analysis zone by a duct (313, 302)surrounding the duct (302) of the liquid being analyzed.
 7. Methodaccording to any one of the foregoing claims, characterized in that thelaser beam is tilted at an angle other than 90 degrees with respect tothe plane formed by the surface of the liquid.
 8. Method according toclaim 7, characterized in that the laser beam is tilted at an anglegreater than 60 degrees with respect to the plane formed by the surfaceof the liquid.
 9. Method according to any one of the foregoing claims,characterized in that the beam emitted by the liquid followingexcitation by the laser beam is collected colinearly to the laser beam.10. Method according to any one of the foregoing claims, characterizedin that in that the gas is argon or helium.
 11. Apparatus for theoptical emission spectroscopy of a liquid excited by a pulsed laserfocused on the surface of this liquid, characterized in that itcomprises: a laser suitable for generating coherent light pulses of apower density of at least one Gw/cm², means capable of generating alaminar jet of liquid to be analyzed, on a length of at least onecentimeter, means capable of generating a laminar jet of gas parallel tothe surface of the liquid to be analyzed, and in contact therewith, ofeliminating the residues of the plasma in suspension in the gas andresulting from a first laser impulse, means capable of focusing thelaser beam into the zone of analysis, onto the surface of the jet ofliquid to be analyzed, a means capable of collecting the light resultingfrom the interaction of the light pulses of the laser with the jet ofthe liquid being analyzed, a spectroscope able to operate in the rangeof frequencies where the rays emitted from the liquid to be analyzed arelocated, and arranged so as to receive the interaction light collectedby the optical fiber bundle, means able to circulate the liquid beinganalyzed, in jet form, and means able to circulate in jet form the gasthat is to be circulated tangentially to the liquid to be analyzed. 12.Apparatus according to claim 11, characterized in that the means able tocollect the light emitted from the liquid to be analyzed is such thatthis light is collected in colinearity with the excitation laser beam,and in that the apparatus comprises an air-tight enclosure in which theliquid to be analyzed and the means able to generate the laminar jet ofgas, the colinearity of the excitation laser beam and the direction ofthe collected light permit the use of only one enclosure porthole forthe laser beam and the collected light.